Pulsed time of flight mass spectrometers



Dec. 28, 1965 F. MELZNER 3,226,543

PULSED TIME OF FLIGHT MASS SPECTROMETER Filed Feb. 20, 1963 2Sheets-Sheet l Fig. 1

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i-r'uabeLrn meL'znev Httorneqs Dec. 28, 1965 F. MELZNER 3,226,543

PULSED TIME OF FLIGHT MASS SPECTROMETER Filed Feb. 20, 1965 2Sheets-Sheet 2 Fig. 3

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WieQeLm MeLzner Fittorness United States Patent 3,226,543 PULSED Till IE0F FLIGHT MASS SPECTROMETERS Friedhelm Melzner, Munich, Germany,assignor to Max- Planclr-Gesellschaft zur Foerderung der Wissenschaftene.V., Goettingen, Germany Filed Feb. 20, 1963, Ser. No. 259,890 Claimspriority, application Germany, Feb. 22, 1962, M 51,908 9 Claims. (Cl.250-413) The present invention relates to dynamic mass spectrometersemploying electrical fields in which the ions oscillate in a potentialtrough and ions of different ratios of charge e to mass m are separatedby the diiference between their times of flight or their oscillationfrequency.

It is an object of the invention to provide a mass spectrometer which isstructurally simple, and which in its preferred embodiment requires onlyvery simple supplementary electronic devices for its operation.

A mass spectrometer in accordance with the invention employs onlyelectrical fields; ions of a particular kind (i.e. of a given ratio ofcharge e to mass m) are arranged to oscillate axially in an evacuatedvessel whereas ions of other kinds are removed from the oscillation pathof the first mentioned ions, so that disturbing space charges cannotbuild up in the region of ion oscillation and impair the sensitivity andpower of resolution of the instrument.

The sensitivity and the power of resolution of mass spectrometersaccording to the invention are relatively high, event at low partialpressures of the vapour or gas under test, despite their simplicity ofconstruction and compactness. A preferred though not an exclusiveapplication of the invention is that of leakage detection.

An object of the invention is to provide a mass spectrometer including alongitudinally extending vacuum tube, means for producing ions in saidtube, means for producing differences of potential along a portion ofthe length of said tube such that there is a relatively positivepotential at one end of said portion and a relatively negative potentialat an intermediate point between the ends of said portion, and means forperiodically producing a relatively positive potential at the other endof said portion.

Another object of the invention is to provide a mass spectrometerincluding a longitudinally extending vacuum tube, means for producingions in said tube, and means for producing differences of potentialalong a portion of the length of said tube such that each end of saidportion is periodically raised to a potential positive with respect to apoint intermediate between said ends.

A further object of the invention is to provide a mass spectrometercomprising a vacuum-tight envelope containing an ion source and a set ofapertured coaxial relatively spaced electrodes, means for biasing atleast one electrode intermediate between the ends of the electrode set,means for biasing an electrode at one end of the set at least duringcertain times by the application of a potential which is more positivethan the potential of the intermediate electrode, and a source of pulsesfor applying to an electrode at the other end of the set a positivepulse train which keeps this electrode at a first potential during afirst part of each pulse cycle and at a second potential more positivethan said first potential during a second part of each pulse cycle.

The vacuum-tight envelope of a mass spectrometer according to theinvention is equipped with means for introducing into it a gas or avapour which it is desired to analyse. Inside the evacuated envelopewhich may preferably be in the form of a tube, there are providedseveral relatively spaced electrodes with coaxial aper- 3,226,543Patented Dec. 28, 1965 tures. The electrodes are connected to lead-inwires which enter the evacuated envelope from the outside throughvacuum-tight seals. In association with a suit able source of potentialthe electrode arrangement is suit table for the generation of an axialelectric potential distribution which is, at least at certain times,more positive at the ends of the electrode system than in its centre. Anion source is located away from the centre of the electrode system,preferably at one end thereof. The more negative potential in the centreof the system accelerates the ions produced by the ion source in thedirection from the source to the centre of the system. Ions of aparticular kind are reflected at the end of the system remote from thesource by a positive potential which preferably exists at this end onlywhen the ions of the said kind are likewise present at said end. Theelectrodes in the centre of the system are preferably biased in such away that the potential throughout an extensive central region is atleast substantially constant. One of the outer electrodes of the systemis connectable to a source of potential pulses supplying a cyclicvoltage which, during the first and preferably longer part of the cycle,remains at a lower positive level, and during a second preferablyshorter part of the cycle, is at a higher positive level.

In one embodiment of the invention electrodes at both ends of theelectrode system may be connected to two output terminals of a singlesource of pulses or to two synchronised separate sources of pulses. Thepositive pulses applied to the electrodes at opposite ends of theelectrode system may appear simultaneously. However, it is preferredthat they should be in antiphase.

In a particularly simple embodiment of the invention a single source ofpulses applies positive pulses only to the end of the electrode systemremote from the ion source. The ion source functions continuously,providing an uninterrupted ion stream. The end of the electrode systemnear the ion source may either be kept permanently at a positivepotential which is adjustable for the detection of a particular kind ofions, or it may be cyclically variable to sweep a given e/m range. Sincethe potential existing at the point where the ions are formed determinestheir energy, it also detremines the velocity at which the ions enterthe middle part of the electrode array which forms a drift path. In thisregion, as has been mentioned, the potential is preferably constant.

The nature of the invention will now be explained in greater detail bydescribing embodiments thereof by reference to the drawings, but theseembodiments are not intended to limit the scope of the invention in anyway. In the drawings:

FIGURE 1 is a simplified representation of a mass spectrometer accordingto the invention,

FIGURE 2 is a graph showing the potentials along the axis 6 of the massspectrometer tube illustrated in FIG- URE l, and

FIGURE 3 is a graph illustrating a particular method of operating a massspectrometer according to FIG- URE 1.

The embodiment of the invention shown in FIGURE 1 comprises a massspectrometer tube in the form of an elongated tubular envelopeconsisting for instance of glass and equipped with conventional means,not shown in the drawing, for the introduction into the tube of a vapouror gas. The interior of the envelope contains an electrode systemcomprising electrodes 101, 102, 103, 104 and in coaxial, relativelyspaced, alignment along an axis 106. The electrodes 101 to 105 areformed with coaxial apertures. The electrodes 101 and 105 are roughlycupshaped, with apertured bottoms, whereas electrodes 102, 103 and 104are cylindrical.

During operation the electrodes 102, 103 and 104 in the middle of thesystem are maintained at least approximately at the same potential, sayat ground potential. The electrodes 101-and/ or 105 may be biased,according to the method of operation of the tube, with DC. and/o1 pulsedpotentials, e.g., by bias generator 122. Electrodes to which pulsedpotential are applied are preferably at the same potential as theelectrodes 102 to 104 during the intervals between the pulses.

In the illustrated embodiment, electrode 101 also serves as the anode ofan electron gun 107 which forms the ion source. Apart from anode 101 theelectron gun 107 also comprises a cathode S and a control electrode 109.The electron gun 107 may be of conventional kind, However, the electrongun 107 may, if desired, be replaced by a different kind of ion sourceof known kind, such as a plasma ion source. Moreover, the electron gunmay be so disposed that the electron beam crosses the tube axis 106.

The manner in which the mass spectrometer according to the inventionfunctions will be described by reference to a particular mode ofoperation and to this end reference .will be made to FIGURES 2 and 3.FIGURE 2 represents the potential distribution along axis 106 of themass spectrometer tube in FIGURE 1, whereas FIGURE 3, on the left-handside, represents an ion timetable. That is to say, the time t is plottedon the ordinate from t downwards, whereas the position s of the ionsalong the tube axis 106 (FIGURE 1) are plotted on the abscissa. Zero onthe abscissa roughly corresponds with the position of electrode 101,whereas the broken line parallel to the ordinate on the right-hand sideof the chart roughly corresponds with the position of electrode 105 atthe other end of the table. The left-hand part of the chart in FIG- URE3 therefore reveals the position of particular ions at particular times.However, the chart has been somewhat simplified by neglectingdecelerations and accelerations occuring when ions are reflected atpositively biased electrodes. The graph on the right-hand side in FIGURE3 is a pulse diagram. This graph shares the same ordinate ie. time twith the chart on the left, but the pulse amplitudes are plotted onaseparate abscissa. For the sake of greater clarity two pulse trains pand p are shown with relatively displaced zeros on the abscissa. Inactual practice the potentials during the intervals between pulses andat the pulse peaks respectively are the same in both pulse trains.

In the mode of operation to be described first, positive pulses areapplied alternately to electrodes 101 and 105. At the time t thepotential of electrode 101 is that of a positive pulse 120 of a firstpulse train p which in the neighbourhood of electrode 101 generates apotential hill 113 (FIGURE 2) with a positive peak at 114. Ions areproduced substantially only during the presence of pulse 120, theelectron beam being emitted only during this period. In the region ofthe peak at 114 ions are thus formed and these are accelerated down thepotential slope 113 towards the centre portion of the electrode system.They enter the region 117, Within the electrodes 102, 103, 104, Wherethe potential is at least substantially constant, at a velocity whichdepends upon the ratio of their charge e to their mass m. The ion driftpath extends through this region 117. As the ions drift through region117, ions having different e/m ratios therefore begin to separatebecause their velocities and hence their transit times are different.

The ions of a particular kind (such as the ions of a gas whose presenceis to be detected in a leakage detector) arrive at electrode 105 at theright-hand end of the electrode system at a time t At this time telectrode 105 has a positive potential due to the arrival of a pulse 118of a second pulse train p This pulse gives rise at the right-hand end ofthe electrode system to a positive potential hill 119 (FIGURE 2) the topof which must be at least as high as the peak at 114 where the ionsoriginated. The ions of a particular e/m ratio which correspond to thetime lag between the pulses of pulse trains p and p are reflected bythis potential bill 119 and return ,through region 117 to electrode 101wherethey arrive at a time t At this time t electrode 101 is againpositively biased by the next positive pulse 120 of the pulse train 17The ions therefore ascend the potential hill 113 to their originalstarting point in the region of the peak at 114 where they change theirdirection to begin a fresh oscillation as above described. Whilst therequired ions are back in the region of the peak at 114 fresh ions arepref erably formed, for instance, by activating the electron gun 107 bythe application of a positive pulse to the control electrode 109.

In other words, the required ions perform a periodic oscillation betweenthe electrodes 101 and 105 in the manner indicated by the full zig-zagline in FIGURE 3. Apart from the ratio e/m of the ions the frequency ofthese oscillations depends upon the length of the drift path 117 andupon the velocity of the ions traversing this path. The velocity of theions in turn is a function of the potential energy at their point oforigin, that is to say it is a function of the potential differencebetween the potential at the point of origin 114 of the ions and thepotential in region 117.

An output signal can be derived from the centre electrode 103. Thefrequency of this signal will then be twice the repetition frequency ofthe reflecting pulses in trains p and p The indicating instrument may bean oscillograph 123, as schematically indicated by a block in thedrawing. The voltage tapped from electrode 103 (after amplification ifrequired) may be used for the vertical deflection of the electron beamof the CRO, whereas its horizontal deflection is synchronised with therepetition frequency of pulse trains p and p The pulses 118 and 120 maybe supplied by two separate synchronised pulse generators or by a singlepulse generator of suitable constructions. Since such arrangements arewell known to a person skilled in the art, it will be unnecessary todescribe them in detail.

The conditions required for the periodical reflection of the requiredions will not cause oscillations of ions of different mass, subject to afew exceptions which will now be considered. Assuming the required ionshave a mass m=4 and a charge e==l (the possibility of multiply chargedions may here be neglected), then ions of lower mass, say m=2, willdrift through region 117 at a greater velocity and therefore arrive atelectrode 105 before the latter has been positively biased by pulse 118.This is indicated by dot-dash line 121. In other Words, upon arrival atthe right-hand end of the electrode system ions of mass 2 will not befaced by a potential barrier and they will therefore exit throughelectrode 105 to be discharged by impinging upon the wall of the tube.

The same also applies to heavier masses, say m=6. Ions having this massdo not arrive at electrode 105 until pulse 118 has already died down.They cannot therefore be reflected. This is indicated by dot-dash line122 in FIGURE 3.

However, the selectively condition is satisfied for ions which have amass equalling (2n+l) times the mass of the required ions, n being apositive integer. Therefore, if the mass spectrometer were used as aleakage detector for helium as the test gas, then the presence of ionshaving the masses 36 and might cause interference. Ions of mass m=36, asshown in FIGURE 3, arrive for the first time at electrode at the time iand they are therefore likewise reflected. However, ions of mass 36could be present only in the form of hydrocarbon ions and suchsubstances can be readily decomposed inside the tube, for instance, by ahot tungsten wire and an electron beam. Indeed, small concentrations ofhydrocarbon can be lowered below the detectability limit by the electronemitting system of the ion source. Alternatively, the effect ofinterference of this kind can be eliminated by electronic means as Willbe hereafter explained.

It will be readily understood that it is possible to sweep the massrange by varying the pulse frequencies p and p and that a massspectrogram can thus be obtained.

The above described working principle may be modified in diverse ways.The following method of operation is particularly simple. Only electrode105 is connected to a pulse generator and the pulse frequency isconstant. Electrode 101 and the preferably continuously operating ionsource have a positive bias which determines the particular type of ionwhich will be selected. The output signal can be derived from thecentral electrode 103, as was done in the previously described method ofoperation. However, in the present instance, the output signal may alsobe derived at detecting means 124 from electrode 101 which then operatesas an influence collector. Alternatively, pulses may be applied toelectrode 101 and the signal derived from electrode 105.

Without taking further steps, the ion source can thus be made to workintermittently even though the operational voltage of the electronemitting system remains constant.

Since the potential energy of the ions at their point of origindetermines their transit time and hence their frequency of oscillationthe magnitude of the DC. voltage and/ or of the pulse voltage applied toelectrode 101 adjacent the ion source 107 is preferably arranged to beadjustable.

For simplifying the electronics of the arrangement, both reflectingelectrodes 101, 105 may be jointly pulsed. However, the reflectionconditions will then be satisfied for ions having only four times theselected mass.

Unwanted ions which as such comply with the existing reflectionconditions (e.g., ions of mass 36 in FIG- URE 3), may, as has beenmentioned, be eliminated by electronic means. One possibility would beto reduce the width of the reflection pulses 118 and/ or 120 to such anextent that, although the light ions to be detected (e.g. ions of mass4) reach the constant potential level along the drift path 117 beforethe pulse (and the potential hill 113 and/ or 119) has died down,nevertheless, the heavier and therefore slower ions (e.g. of mass 36)will still be in the course of descent when the potential slopecollapses. The heavier ions will then fail to attain their full energiesand velocities, and will reach the other end of the tube too late forreflection.

Another possibility lies in applying to the ions at the end of theelectrode system remote from the ion source a very short voltage pulsesuch as to apply to them a momentum which is twice as large as, and ofopposite sign to, the momentum mv of the selected ions of mass m whicharrive at velocity v. Mathematically speaking: mv (momentum of thearriving selected ion)+(2 mv) (momentum imported by the electricalpulse)=mv (momentum of the reflected ion). This ensures that only ionsof the required mass will return to the drift path 117 at the samevelocity as before. Ions of heavier mass re turn at a slower speed andare therefore unable to ascend to the potential level of their originalstarting point. Ions of lighter weight are propelled beyond the peak at114 and are thus likewise eliminated.

The magnitude of the reflecting pulse may be suitably chosen to drivethe required ions just over the peak 114 to enable them then to enter asecondary electron multiplier not shown in the drawing. In such a casethe electron stream is directed perpendicularly across the tube axis.Pulse-reflected lighter weight ions which are also able to ride over thepeak 114 can be recognised by their greater energy. They can be detectedin an anticoincidence device or eliminated from the desired signal bysubtracting a corresponding direct current. The reflection of the ionsby the described sign-reversal of their momenta is particularlyadvantageous when combined with the above-described simple method ofoperation. When the ion source is continuously in operation undesirableions are always reflected, which then produce an interfering signal ofthe same frequency as the desired signal. Pulse-reflected heavier ionscannot now reach electrode 101 which functions as an influence collectorbecause they accept too little energy in reflection.

In reflection by reversal of momentum the resultant sensitivity topartial pressures is particularly high and this mode of operation istherefore especially useful for leakage detection. If helium is the testgas, only hydrogen ions in addition to helium ions can in practiceinfluence a signal in electrode 101 or, when a multiplier is used,overcome the peak at 114. However, the hydrogen partial pressure isgenerally so small that interference need not be feared even if theabove-mentioned arrangements for dealing with lighter ions have not beenmade.

If the ion source is pulsed, it is desirable that all newly formed ionsshould start at the same time. This can be achieved by providing anadditional potential barrier 116 (FIGURE 2) with the aid of an auxiliaryelectrode 115 and thereby preventing the freshly formed ions fromfalling into the drift path until they are all released simultaneouslyat the end of the ion forming period. The auxiliary electrode 115 may beannular and located inside electrode 101.

The potential pulses applied to the pulsed electrode may sometimes giverise to an interference signal at the electrode from which the outputsignal is derived. Such interference can be eliminated as follows:

A compensating electrode may be interposed between the pulsed electrodeand the output signal electrode and a pulse in antiphase of suitableamplitude may be ap plied thereto.

The interfering potentials at the output signal electrode can beneutralised in a manner known to the art.

Interference potentials will cancel each other out at the centreelectrode 103 if electrodes 101 and 105 are simultaneously pulsed inantiphase, that is to say if a positive reflecting pulse is applied toone of the electrodes at the same time as a negative pulse of likeamplitude is applied to the other electrode. This is indicated in FIGURE2 by the dot-dash curves in FIGURE 2.

Finally, the ion source may be modulated with an arbitrary low frequencyin such manner that the desired signal can be recognised by itsmodulation and isolated.

For focusing the ions radially in the tube axis an axially directed,substantially constant magnetic field may be provided in all theembodiments which have been described.

'In the last described mode of operation region 117 may also includearrangements additional to those already described for improving ionselectivity. For instance, a magnetic ion optical system or a Paulfour-pole field may be provided. However, these are known steps andrequire no further description.

What we claim is:

1. .A mass spectrometer operative in a pulse cycle and comprising, incombination: a vacuum-tight envelope; at a set of apertured electrodesspaced along a common longitudinal axis within said envelope, at leastone of said electrodes intermediate the ends of said electrode setforming an intermediate ion drift region; an ion source for providingions at one end of said drift region; means for biass-ing said ionsource positively with respect to said drift region so that ionsproduced by said source are accelerated along the negative potentialgradient thus formed toward said drift region and continue to drifttherethrough to the other end of said drift region; pulse generatormeans connected to an electrode at a second end of said drift region,opposite said one end, for delivering periodic pulses which are positivewith respect to the potential of said drift region and which have suchrepetition rate and duration that of the accelerated ions which start ata given point in time, only those ions which have a selected charge/massratio are repelled back into said drift region; and means for detectingsaid ions of selected charge/ mass ratio which are periodicallyreflected by said positive biasses at the ends of said drift region.

2. A mass spectrometer as defined in claim 1 wherein said means forbiassing said ion source produces a positive DC. bias.

3. A mass spectrometer as defined in claim 1 wherein said means forbiassing said ion source produces a periodic pulse bias having the samerepetition rate as the pulses produced by said generator means.

4. A mass spectrometer as defined in claim 3 wherein both said biassingmeans and said pulse generator produce alternatively pulses of positiveand negative polarity.

5. A mass spectrometer as defined in claim 3 including means forproviding a positive potential barrier directly in front of said ionsource at all times except when the ions produced by said source are tobe released, so that all ions so produced leave said sourcesimultaneously.

6. A mass spectrometer according to claim 3, wherein the pulses appliedto the electrode at the end of the electrode set remote from the ionsource are very short in relation to the repetition rate and theiramplitude is such as to apply a momentum to the arriving ions which inmagnitude is equal to twice the momentum of the arriving desired ionsbut of opposite sign.

7. A mass spectrometer operative in a pulse cycle and comprising, incombination: a vacuum-tight envelope; at set of apertured electrodesspaced along a common longitudinal axis within said envelope, at leastone of said electrodes intermediate the ends of said electrode setforming an intermediate ion drift region; an ion source for providingions at one end of said drift region; a first source of periodic pulsesfor biassing said ion source positively with respect to said driftregion, so that ions produced by said source are acceleratedperiodically along the negative potential gradient thus formed towardsaid drift region and continue to drift therethrough to the other end ofsaid drift region; means for biassing positively with respect to saiddrift region an electrode at said other end of said drift region,opposite said one end, so that the ions incident at said electrode arerepelled back into said drift region; and means for detecting ions :of aselected charge/mass ratio which are periodically reflected by saidpositive biasses at the ends of said drift region.

8. A mass spectrometer as defined in claim 7 wherein said means forbiassing said electrode at the other end of said drift regionperiodically delivers pulses which are positive with respect to saiddrift region and which have a repetition rate and duration such that ofthe accelerated ions which started during the period in which said ionsource was positively biassed by a pulse of said first pulse source,only those ions which have a selected charge/ mass ratio are repelled tosaid drift region, said first source of periodic pulses and said meansfor biassing being arranged to provide biassing .at the same time.

9. A mass-spectrometer operative in a pulse cycle and comprising, incombination: a vacuum-tight envelope; a set of ape'rtured electrodesspaced along a common longitudinal axis within said envelope; an ionsource for providing ions at one end of said axis; means for biassing atleast one electrode intermediate between the ends of the electrode setto a given potential for providing an ion drift region; pulse generatormeans connected to a first electrode at said one end of the electrodeset for biassing said first electrode, at least during certain portionsof said cycle, to a potential which is more positive than that of saidintermediate electrode for preventing ions from passing said firstelectrode; pulse generator means connected to a second electrode at theother end of said electrode set from said first electrode forperiodically raising the potential of said second electrode to a valueabove said given potential to deflect selected ions back along thelongitudinal axis of said spectrometer; said pulse generator means beingarranged to alternately bias said first electrode and raise thepotential of said second electrode and means for detecting thoseselected ions so deflected.

References Cited by the Examiner UNITED STATES PATENTS 2,642,535 6/1953Schroeder 25041.9 2,772,364 11/1956 Washburn 250-419 2,798,162 7/1957Hendee 25041.9 2,810,075 10/1957 Hall et al 25041.9 2,908,816 40/195 9Le Poole 25041.9 2,938,116 5/1960 Benson et a1 250-41.9

RALPH G. NILSON, Primary Examiner.

9. A MASS-SPECTROMETER OPERATIVE IN A PULSE CYCLE AND COMPRISING, INCOMBINATION: A VACUUM-TIGHT ENVELOPE; A SET OF APERTURED ELECTRODESSPACED ALONG A COMMON LONGITUDINAL AXIS WITHIN SAID ENVELOPE; AN IONSOURCE FOR PROVIDING IONS AT ONE END OF SAID AXIS; MEANS FOR BIASING ATLEAST ONE ELECTRODE INTERMEDIATE BETWEEN THE ENDS OF THE ELECTRODE SETTO A GIVEN POTENTIAL FOR PROVIDING AN ION DRIFT REGION; PULSE GENERATORMEANS CONNECTED TO A FIRST ELECTRODE AT SAID ONE END OF THE ELECTRODESET FOR BIASING SAID FIRST ELECTRODE, AT LEAST DURING CERTAIN PORTIONSOF SAID CYCLE, TO A POTENTIAL WHICH IS MORE POSITIVE THAN THAT OF SAIDINTERMEDIATE ELECTRODE FOR PREVENTING IONS FROM PASSING SAID FIRSTELECTRODE; PULSE GENERATOR MEANS CONNECTED TO A SECOND ELECTRODE AT THEOTHER END OF SAID ELECTRODE SET FROM SAID FIRST ELECTRODE FORPERIODICALLY RAISING THE POTENTIAL OF SAID SECOND ELECTRODE TO A VALUEABOVE SAID GIVEN POTENTIAL TO DEFLECT SELECTED IONS BACK ALONG THELONGITUDINAL AXIS OF SAID SPECTROMETER; SAID PULSE GENERATOR MEANS BEINGARRANGED TO ALTERNATELY BIAS SAID FIRST ELECTRODE AND RAISE THEPOTENTIAL OF SAID SECOND ELECTRODE AND MEANS FOR DETECTING THOSESELECTED IONS SO DEFLECTED.